Two‐tone suppression in the auditory nerve of the cat: Suppression thresholds and rate of growth

Abstract

Our previous work has been directed at elucidating the behavior of the two‐tone suppression phenomena when both tones are excitatory. Analyzing the phase‐locked response of fibers possessing low characteristic frequency (CF), we have shown [E. Javel, J. Acoust. Soc. Am. Suppl. 1 65, S83(A) (1979)] that suppression exists throughout an auditory nerve fiber's response area and that the ability of an excitatory tone to suppress the response to another excitatory tone is similar to a nonexcitatory tone's ability to suppress. For a given suppressor‐tone intensity, suppression is maximal when the suppressor tone lies near fiber CF, and suppression magnitude is reduced on either side of CF. The contours relating suppression magnitude to frequency are similar in shape, but not in extent, to the isointensity response contours observed in determinations of response area. The “threshold” for suppression is systematically related to fiber sensitivity at the suppressor‐tone frequency, but it is always greater or equal to the pure‐tone threshold at that frequency. Threshold for suppression is not related to the threshold for the tone being suppressed. Once the threshold is exceeded, suppression for both excitatory and nonexcitatory tones grows with increasing suppressor‐tone intensity in a manner consistent with that described by Sachs and Abbas [J. Acoust. Soc. Am. 60, 1157–1163 (1976)]. The final slope of the curve relating suppression magnitude to suppressor‐tone intensity is near unity for suppressor frequencies near fiber CF, but is usually greater than one for suppressors much lower in frequency than CF and less than one for suppressors higher in frequency than CF. Suppression does not saturate at high intensities, but in many instances its magnitude is obscurred by the “notch” often seen in single‐tone rate‐intensity functions [Kiang et al., J. Acoust. Soc. Am. 46, 106(A) (1969)], i.e., the “notch” cannot be suppressed. [Work supported by NIH.]